be thin on top
To be balding. This phrase refers to thinning hair on the top of one's head. I should have known I would be thin on top by 30—none of the older men in my family have much hair.
get thin on top
To become bald. This phrase refers to thinning hair on the top of one's head. I should have known I would get thin on top by 30—none of the older men in my family have much hair.
be/get thin on ˈtop
(informal) without much hair on the head; be/go bald: Max is only 30 but he’s already getting a bit thin on top.